81 research outputs found
Evolutionary prisoner's dilemma games with optional participation
Competition among cooperators, defectors, and loners is studied in an
evolutionary prisoner's dilemma game with optional participation. Loners are
risk averse i.e. unwilling to participate and rather rely on small but fixed
earnings. This results in a rock-scissors-paper type cyclic dominance of the
three strategies. The players are located either on square lattices or random
regular graphs with the same connectivity. Occasionally, every player
reassesses its strategy by sampling the payoffs in its neighborhood. The loner
strategy efficiently prevents successful spreading of selfish, defective
behavior and avoids deadlocks in states of mutual defection. On square
lattices, Monte Carlo simulations reveal self-organizing patterns driven by the
cyclic dominance, whereas on random regular graphs different types of
oscillatory behavior are observed: the temptation to defect determines whether
damped, periodic or increasing oscillations occur. These results are compared
to predictions by pair approximation. Although pair approximation is incapable
of distinguishing the two scenarios because of the equal connectivity, the
average frequencies as well as the oscillations on random regular graphs are
well reproduced.Comment: 6 pages, 7 figure
Motion of influential players can support cooperation in Prisoner's Dilemma
We study a spatial Prisoner's dilemma game with two types (A and B) of
players located on a square lattice. Players following either cooperator or
defector strategies play Prisoner's Dilemma games with their 24 nearest
neighbors. The players are allowed to adopt one of their neighbor's strategy
with a probability dependent on the payoff difference and type of the given
neighbor. Players A and B have different efficiency in the transfer of their
own strategy therefore the strategy adoption probability is reduced by a
multiplicative factor (w < 1) from the players of type B. We report that the
motion of the influential payers (type A) can improve remarkably the
maintenance of cooperation even for their low densities.Comment: 7 pages, 7 figure
Phase transition and selection in a four-species cyclic Lotka-Volterra model
We study a four species ecological system with cyclic dominance whose
individuals are distributed on a square lattice. Randomly chosen individuals
migrate to one of the neighboring sites if it is empty or invade this site if
occupied by their prey. The cyclic dominance maintains the coexistence of all
the four species if the concentration of vacant sites is lower than a threshold
value. Above the treshold, a symmetry breaking ordering occurs via growing
domains containing only two neutral species inside. These two neutral species
can protect each other from the external invaders (predators) and extend their
common territory. According to our Monte Carlo simulations the observed phase
transition is equivalent to those found in spreading models with two equivalent
absorbing states although the present model has continuous sets of absorbing
states with different portions of the two neutral species. The selection
mechanism yielding symmetric phases is related to the domain growth process
whith wide boundaries where the four species coexist.Comment: 4 pages, 5 figure
Dynamic instabilities induced by asymmetric influence: Prisoners' dilemma game on small-world networks
A two-dimensional small-world type network, subject to spatial prisoners'
dilemma dynamics and containing an influential node defined as a special node
with a finite density of directed random links to the other nodes in the
network, is numerically investigated. It is shown that the degree of
cooperation does not remain at a steady state level but displays a punctuated
equilibrium type behavior manifested by the existence of sudden breakdowns of
cooperation. The breakdown of cooperation is linked to an imitation of a
successful selfish strategy of the influential node. It is also found that
while the breakdown of cooperation occurs suddenly, the recovery of it requires
longer time. This recovery time may, depending on the degree of steady state
cooperation, either increase or decrease with an increasing number of long
range connections.Comment: 5 pages, 6 figure
Asteroid Confusions with Extremely Large Telescopes
Asteroids can be considered as sources of contamination of point sources and
also sources of confusion noise, depending whether their presence is detected
in the image or their flux is under the detection limit. We estimate that at
low ecliptic latitudes, ~10,000--20,000 asteroids/sq. degree will be detected
with an E-ELT like telescope, while by the end of Spitzer and Herschel
missions, infrared space observatories will provide ~100,000 serendipitous
asteroid detections. The detection and identification of asteroids is therefore
an important step in survey astronomy.Comment: 8 pages, 4 figures, accepted by Earth, Moon and Planets, ELT
Conference (Elba, 2009 Sept.) S
Disordered Environments in Spatial Games
The Prisoner's dilemma is the main game theoretical framework in which the
onset and maintainance of cooperation in biological populations is studied. In
the spatial version of the model, we study the robustness of cooperation in
heterogeneous ecosystems in spatial evolutionary games by considering site
diluted lattices. The main result is that due to disorder, the fraction of
cooperators in the population is enhanced. Moreover, the system presents a
dynamical transition at , separating a region with spatial chaos from
one with localized, stable groups of cooperators.Comment: 6 pages, 5 figure
Cooperation and its evolution in growing systems with cultural reproduction
We explore the evolution of cooperation in the framework of the evolutionary
game theory using the prisoner's dilemma as metaphor of the problem. We present
a minimal model taking into account the growing process of the systems and
individuals with imitation capacity. We consider the topological structure and
the evolution of strategies decoupled instead of a coevolutionary dynamic. We
show conditions to build up a cooperative system with real topological
structures for any natural selection intensity. When the system starts to grow,
cooperation is unstable but becomes stable as soon as the system reaches a
small core of cooperators whose size increase when the intensity of natural
selection decreases. Thus, we reduce the emergence of cooperative systems with
cultural reproduction to justify a small initial cooperative structure that we
call cooperative seed. Otherwise, given that the system grows principally as
cooperator whose cooperators inhabit the most linked parts of the system, the
benefit-cost ratio required for cooperation evolve is drastically reduced
compared to the found in static networks. In this way, we show that in systems
whose individuals have imitation capacity the growing process is essential for
the evolution of cooperation.Comment: 16 pages, 2 figures. arXiv admin note: substantial text overlap with
arXiv:1111.247
Mixed state properties of superconducting MgB2 single crystals
We report on measurements of the magnetic moment in superconducting MgB2
single crystals. We find \mu_0H_{c2}^c(0) = 3.2 T, \mu_0H_{c2}^{ab}(0) = 14.5
T, \gamma = 4.6, \mu_0H_c(0) = 0.28 T, and \kappa(T_c) = 4.7. The standard
Ginzburg-Landau and London model relations lead to a consistent data set and
indicate that MgB2 is a clean limit superconductor of intermediate coupling
strength with very pronounced anisotropy effects
Analysis of a spatial Lotka-Volterra model with a finite range predator-prey interaction
We perform an analysis of a recent spatial version of the classical
Lotka-Volterra model, where a finite scale controls individuals' interaction.
We study the behavior of the predator-prey dynamics in physical spaces higher
than one, showing how spatial patterns can emerge for some values of the
interaction range and of the diffusion parameter.Comment: 7 pages, 7 figure
Selenocompounds as Novel Antibacterial Agents and Bacterial Efflux Pump Inhibitors
Bacterial multidrug resistance is becoming a growing problem for public health, due to
the development and spreading of bacterial strains resistant to antimicrobials. In this study, the
antibacterial and multidrug resistance reversing activity of a series of seleno-carbonyl compounds
has been evaluated. The effects of eleven selenocompounds on bacterial growth were evaluated in
Staphylococcus aureus, methicillin resistant S. aureus (MRSA), Enterococcus faecalis, Escherichia coli, and
Chlamydia trachomatis D. The combination effect of compounds with antibiotics was examined by the
minimum inhibitory concentration reduction assay. Their efflux pump (EP) inhibitory properties
were assessed using real-time fluorimetry. Relative expressions of EP and quorum-sensing genes
were studied by quantitative PCR. Results showed that a methylketone selenoester had remarkable
antibacterial activity against Gram-positive bacteria and potentiated the activity of oxacillin in MRSA.
Most of the selenocompounds showed significant anti-chlamydial effects. The selenoanhydride
and the diselenodiester were active inhibitors of the AcrAB-TolC system. Based on these results
it can be concluded that this group of selenocompounds can be attractive potential antibacterials
and EP inhibitors. The discovery of new derivatives with a significant antibacterial activity as novel
selenocompounds, is of high impact in the fight against resistant pathogen
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